Sometimes we use the "+5_iso" power port (as example) in a sheet to symbolize the output of a isolated power supply and its connections of the others components.
Sometimes, even, this sheet is used as a hierarchical sheet more than one time to symbolize a PCB / schematic module (one example can be saw at https://github.com/hildogjr/easyInverter/blob/master/Assemblies/2017-08-16%20Full%20inverter.PDF, where the "Isolated power supply" sheet is used to supply the power drive of power MOSFETs of a H-Bridge inverter). But, in this case, the "+5_iso"/"GND_iso" remain as a local power definition (the Altium software allow this configuration to the project).
It will be interesting if the Eeschema have this feature too, allow the configuration of "power port" be local in the sheet internal of the "Hierarchical Sheet".
Other think that I thought that could help this is, the others EDA softwares deal with the power pin as just a graphical object (not in the components libraries) that set the wire net name.
This could help that the most of the time we keep creating new "power pins" in the library to different voltages values, but do not change the symbol it self.
So, the power pin could be graphical elements with the propriety NET, that could be modified (+VDC, -VDC, +5V, GND, GNDD, GNDA, ...). And so, in the hierarchical page it could inherit the propriety of the project to make them NETs global or local.